The present invention generally relates to a system for determining wheel speed and more particularly relates to a method and a device for determining correction values for use in determining vehicle wheel speeds.
Wheel speeds normally are sensed by wheel sensors. Wheel sensors of this type, normally, are so designed as to sense angular speeds, for example, by monitoring on a sensor the passage of markings that move along with the wheel, thereby obtaining a pulse sequence the frequency of which rises the wheel speed increases.
The wheel speed, in technical language also called circumferential speed of the wheel, is related to the rotating or angular speed by the mathematical relationship: v=xcfx89. r, wherein v is the wheel speed (more precisely speaking, the circumferential speed of the wheel); xcfx89 is the angular speed of the wheel and r is the tire radius. The tire radius is, therefore, integrated into the computation of the wheel speed. The tire radius can be determined, for example, from manufacturer"" data by applying an average value within the manufacturing tolerances. As the tire radii can vary on the individual wheels of a vehicle, for example, due to wear, manufacturing tolerances, use of tires from different manufacturers etc., and as the precise values of the radii are not known, the wheel speed can be computed from the directly sensed (and, optionally, filtered) angular speed of the wheel considered by way of approximation only.
In comparatively precise slip and stability controls but also for algorithms of vehicle state detection employing, for example, the wheel slip or the wheel speed differences etc., it is, therefore, mandatory to eliminate the negative influence of different fire radii. Consequently, correction values will have to be determined for each individual wheel by which the corresponding wheel speed can be corrected, as the wheel speeds, in the majority of cases, are determined on the assumption that the wheels are of exactly the same tire radii.
DE 4 424 318 C2 teaches a traction slip control system stipulating conditions for determining correction factors. The vehicle sides (left or right) are separately considered. A tire tolerance computation is permitted if predetermined conditions with respect to the butterfly throttle angle, on the one hand, and with respect to the relationship of the speeds of the wheels of actuated and non-actuated axles on the vehicle side under consideration, on the other hand, have been complied with.
This prior art method involves the disadvantage that by comparing the non-corrected wheel speeds, the scaling conditions in a substantially deviating wheel, frequently, are fulfilled, thereby substantially affecting the accuracy of a subsequent evaluation method. Moreover, no criteria are quoted for scaling the wheel speeds of the left-hand vehicle side to the right-hand vehicle side.
DE 3 718 421 reveals a method for cornering detection and for determining a correction factor by which the relationship of the tire radii of the non-actuated axle is determined. This state-of-the-art method proceeds from the assumption that the cornering-dependent influence on the determination can be eliminated through a long-time filtration of between 30 and 40 seconds. However, when dealing with winding roads this assumption will either yield inaccurate results or substantially higher time constants should be selected for filtering, which would involve extended instruction times.
U.S. Pat. No. 5,682,333 describes a method of scaling the wheel speeds for a vehicle, wherein the scaling factors for the wheels are determined to detect scaled and corrected speeds. In a first step of the this method, a rough-stage determination of scaling factors is carried out by means of fast and rough scaling, if non-cornering, a predetermined minimum speed and a low vehicle acceleration, at best, have been detected. Subsequently, a fine-stage scaling factor is determined by fine scaling, wherein either each wheel of an axle is scaled to the respective wheel on the same side of the other axle, if a low driving moment exceeding of a minimum speed have been detected, or each wheel on one side is scaled to the respectively opposite wheel of the same axle, if a higher driving moment, a moderate cornering, at best, and the exceeding of a minimum speed have been detected.
It is the object of the present invention to provide a method and a device for determining correction values for the wheel speeds of a vehicle, enabling a rapid, reliable and accurate determination of correction values for all wheels of the vehicle.
As in state-of-the-art slip control functions, wheel speeds, frequently are compared axle- or side-wise, it is important for the wheel speed values to be comparable axle-wise (at the front axle or at the rear axle) and side-wise (right-hand side, left-hand side). Virtually, this will result in the condition that all wheel speeds of the vehicle must be comparable with one another in pairs so that a corresponding set of factors of corrections is to be determined. If there is no need to precisely determine the absolute value of the wheel speeds it may be adequate to select a factor of correction (preferably a xe2x80x9croundedxe2x80x9d value) and to determine the values of correction for the rest of the wheels in relation thereto.
First, individual scalings are performed for at least one vehicle side (the left-hand wheels and right-hand wheels, respectively) and for at least one vehicle axle (the wheels on the front axle and the wheels on the rear axle). Proceeding from these individual scaling results the final scaling is then carried out to determine the afore-mentioned values of correction.
Preferably, individual scalings are effected for the left-hand- and right-hand-sides of the vehicle and for the vehicle axle non-actuated (or deemed or identified as non-actuated). After such scaling having been carried out, the complete set of correction values is determined for all wheels of the vehicle. Scaling for one vehicle side for one vehicle axle is effected by evaluating the wheel speeds sensed for the wheels on that side and on that axle, respectively. Evaluation can be in real time (immediate processing the momentarily sensed values) or in reference to temporarily stored values.